This invention relates to a humidity sensing device and more particularly to a humidity sensor which detects humidity by measuring the change in the electrical characteristic of an element corresponding to the ambient humidity.
Fields requiring humidity measurement and humidity control have been increasing recently and the importance of a humidity sensor is widely recognized. There are several types of humidity sensors which detect humidity by measuring the change in the electrical characteristic of an element corresponding to the ambient humidity, including electrolytic, metallic, polymeric and ceramic humidity sensors. These humidity sensors have each been extensively studied and polymeric and ceramic humidity sensors have been put to practical use. Each of these sensors measures the humidity level by measuring the change in the resistance of an element or the change in the electrostatic capacity of an element as the element absorbs or releases moisture. A resistance-change type humidity sensor is a humidity sensor which detects humidity by measuring the change in the resistance of an element corresponding to the ambient humidity. A capacitance-change type humidity sensor is a humidity sensor which detects humidity by measuring the change in the electrostatic capacity of an element corresponding to the ambient humidity.
Many of the conventional resistance-change type humidity sensors have such high resistance at low humidity that it is difficult to measure low humidity with high accuracy. In order to construct a humidity sensor which is able to measure low humidity with high accuracy, high quality circuitry and a highly accurate mounting technique are required, leading to an increase in manufacturing cost.
Generally, in a resistance-change type humidity sensor, the logarithm of resistance changes linearly with respect to the change in relative humidity. If the linearity is good, a logarithm amplifier is capable of compensating the linearity. In conventional humidity sensors, however, the linearity is poor. Thus, a complicated linearity compensation circuit is required to produce a highly accurate hygrometer.
Generally, in a resistance-change type humidity sensor, the greater the rate of change in the resistance between low humidity and high humidity, the greater the sensitivity. However, when a hygrometer is produced it is difficult to provide the dynamic range of the measuring circuit if the rate of change in the resistance between low humidity and high humidity is too large. Thus, the desirable rate of change in the resistance at a relative humidity of 0 to 100% is about 1 to 3 figures. In conventional humidity sensors, however, the rate of change is large, and in order to produce a highly accurate hygrometer, high quality circuitry and a highly accurate mounting technique are required.
In a capacitance-change type humidity sensor, the linearity of change in electrostatic capacity with respect to relative humidity is poor. Thus, a complicated linearity compensation circuit is required to produce a highly accurate hygrometer. In addition, many capacitance-change type humidity sensors have poor stability at high humidity. Consequently, it is difficult to measure high humidity with high accuracy.
In conventional humidity sensors moisture sensitivity is highly dependent on the temperature and a temperature compensation circuit is required. When temperature dependence is represented by a simple function, it is not necessary to have a complicated temperature compensation circuit. However, in conventional humidity sensors dependence of moisture sensitivity on temperature is not represented by a simple function and a complicated temperature compensation circuit is required to produce a highly accurate hygrometer. Although a complicated temperature compensation circuit may be provided, complete temperature compensation is difficult in a place in which the change in temperature is large. This is due to a difference in thermal response between the humidity sensor and the temperature sensor, or a difference in the location of the temperature sensor and the humidity sensor. In other words, as long as moisture sensitivity of the humidity sensor is dependent on the temperature, accurate humidity measurement remains difficult.
As the prior art illustrates, a highly accurate hygrometer is difficult to produce. A conventional humidity sensor is expensive to manufacture since it requires high quality circuitry and a highly accurate mounting technique. Additionally, inspection and control of the humidity sensor require a large amount of time. In addition, the circuit demands a large volume of power. In view of this, it has not been possible to produce a long life humidity sensor powered by a battery.
With many polymeric humidity sensors, the reliability deteriorates at high temperature and high humidity. The deterioration is especially prominent when an organic solvent is used. Some ceramic humidity sensors have a heating refreshment mechanism to reverse the deterioration by heating an element several hundred degrees (between about 300.degree. and 800.degree. C.) for a determined period. The heating refreshment mechanism can reduce a change in the characteristic with time. This type of humidity sensor, however, cannot be used in an environment in which a combustible gas or dust exists, because there is a danger of explosion or fire when the element is heated to a high temperature. Accordingly, it is not an exaggeration to say that there is no satisfactory humidity sensor in the prior art.
Accordingly, it is desirable to provide an improved humidity sensor which eliminates these problems associated with prior art devices and accurately measures humidity even in a severe environment.